In recent years, with the electric power shortage becoming more serious, rapid introduction of natural energy, such as wind power generation and solar power generation, and stabilization of electric power systems have become issues to be addressed globally. As one of the measures to address the issues, installation of large-capacity storage batteries to achieve, for example, smoothing of output fluctuation, storage of surplus electricity, and load leveling has been receiving attention.
One of such large-capacity storage batteries is a redox flow battery (hereinafter, may be referred to as an “RF battery”). The RF battery has characteristics such as 1) ease of capacity increase to a megawatt (MW) level, 2) a long life, and 3) capability of accurately monitoring the state of charge of the battery, and is expected to be the most suitable storage battery for stabilization of electric power systems.
An RF battery mainly includes a battery cell portion including a positive electrode, a negative electrode, and a membrane disposed between the two electrodes, in which a positive electrode electrolyte and a negative electrode electrolyte are supplied, and charging and discharging are performed. Typically, a system is constructed for use, in which the battery cell portion and each of the tanks that store the electrolytes are connected with a pipe, and by providing a pump on the pipe, each electrode electrolyte is circulated and supplied to the battery cell portion.
Electrolytes used in RF batteries contain, as active materials, metal elements whose valence is changed by oxidation-reduction. Recently, as described in PTL 1 and 2, typically used are vanadium-based electrolytes containing vanadium ions as an active material for both positive and negative electrodes.